tpm_tis_spi: Don't send anything during flow control

During flow control we are just reading from the TPM, yet our spi_xfer
has the tx_buf and rx_buf both non-NULL which means we're requesting a
full duplex transfer.

SPI is always somewhat of a full duplex protocol anyway and in theory
the other side shouldn't really be looking at what we're sending it
during flow control, but it's still a bit ugly to be sending some
"random" data when we shouldn't.

The default tpm_tis_spi_flow_control() tries to address this by
setting 'phy->iobuf[0] = 0'.  This partially avoids the problem of
sending "random" data, but since our tx_buf and rx_buf both point to
the same place I believe there is the potential of us sending the
TPM's previous byte back to it if we hit the retry loop.

Another flow control implementation, cr50_spi_flow_control(), doesn't
address this at all.

Let's clean this up and just make the tx_buf NULL before we call
flow_control().  Not only does this ensure that we're not sending any
"random" bytes but it also possibly could make the SPI controller
behave in a slightly more optimal way.

NOTE: no actual observed problems are fixed by this patch--it's was
just made based on code inspection.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>

drivers/char/tpm/tpm_tis_spi_main.c

@@ -53,8 +53,6 @@ static int tpm_tis_spi_flow_control(struct tpm_tis_spi_phy *phy,
 
 	if ((phy->iobuf[3] & 0x01) == 0) {
 		// handle SPI wait states
-		phy->iobuf[0] = 0;
-
 		for (i = 0; i < TPM_RETRY; i++) {
 			spi_xfer->len = 1;
 			spi_message_init(&m);
@@ -104,6 +102,8 @@ int tpm_tis_spi_transfer(struct tpm_tis_data *data, u32 addr, u16 len,
 		if (ret < 0)
 			goto exit;
 
+		/* Flow control transfers are receive only */
+		spi_xfer.tx_buf = NULL;
 		ret = phy->flow_control(phy, &spi_xfer);
 		if (ret < 0)
 			goto exit;
@@ -113,9 +113,8 @@ int tpm_tis_spi_transfer(struct tpm_tis_data *data, u32 addr, u16 len,
 		spi_xfer.delay.value = 5;
 		spi_xfer.delay.unit = SPI_DELAY_UNIT_USECS;
 
-		if (in) {
-			spi_xfer.tx_buf = NULL;
-		} else if (out) {
+		if (out) {
+			spi_xfer.tx_buf = phy->iobuf;
 			spi_xfer.rx_buf = NULL;
 			memcpy(phy->iobuf, out, transfer_len);
 			out += transfer_len;